1. Technical Field
This invention relates generally to multi-cell lithium chemistry battery systems, and, more particularly, to methods and apparatus for balancing such cells.
2. Description of the Related Art
Rechargeable, multi-cell battery systems have been known for decades, and have been based on various chemistries including lead acid (PbA), nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (LiIon) and lithium polymer (LiPo). A key performance aspect of each battery technology relates to how charging (and overcharging) is accomplished, and how inevitable cell imbalances are addressed.
Conventionally, cell-to-cell imbalances in lead-acid batteries, for example, have been solved by controlled overcharging. Lead-acid batteries can be brought into overcharge conditions without permanent cell damage, inasmuch as the excess energy is released by gassing. This gassing mechanism is the natural method for balancing a series string of lead acid battery cells. Other chemistries, such as NiMH, exhibit similar natural cell-to-cell balancing mechanisms.
Lithium ion and lithium polymer battery chemistries, however, cannot be overcharged without damaging the active materials. The electrolyte breakdown voltage is precariously close to the fully charged terminal voltage. Therefore, careful monitoring and controls must be implemented to avoid any single cell from experiencing an over voltage due to excessive charging. Because a lithium battery cannot be overcharged, there is no natural mechanism for cell equalization.
Even greater challenges exist depending on whether the battery system is a single cell or multiple cells. Single lithium-based cells require monitoring so that cell voltage does not exceed predefined limits of the chemistry. Series-connected lithium cells, however, pose a more complex problem; each cell in the string must be monitored and controlled. Even though the system voltage may appear to be within acceptable limits, one cell of the series string may be experiencing damaging voltage due to cell-to-cell imbalances. Based on the foregoing, without more, the maximum usable capacity of the battery system may not be obtained because during charging, an out-of-balance cell may prematurely approach the end of charge voltage and trigger the charger to turn off (i.e., to save that cell from damage due to overcharge as explained above).
One approach taken in the art to address the foregoing problem involves the concept of cell balancing. Cell balancing is useful to control the higher voltage cells until the rest of the cells can catch up. In this way, the charger is not turned off until the cells reach the end-of-charge (EOC) condition more or less together. More specifically, the cells are first charged, and then, during and at the end-of-charging, the cells are balanced.
One example of a cell balancing approach involves energy dissipation. A shunt resistor, for example, may be selectively engaged in parallel with each cell. This approach shunts the excess energy as each cell reaches an end-of-charge condition, resulting in the system becoming more active as the cells reach full charge. During the moments preceding full charge in a system with n total cells, (n−1) cells are dissipating equalization energy as the last cell approaches end-of-charge. This condition results in a buildup of waste energy in the form of heat, which can trigger thermal controls (i.e., discontinuing the charging temporarily until the temperature comes down). These controls extend the overall charge time for the battery system.
Accordingly, there is a need for a method and apparatus for operating a battery system that minimizes or eliminates one or more of the problems as set forth above.